1. Field of the Invention
The present invention relates generally to a method of controlling the current of a Switched Reluctance Motor (SRM), and, more particularly, to a method of controlling the current of an SRM using an inverter circuit including a first switching element, a second switching element, a first diode, a second diode and a reactor, which is configured such that the first switching element and the first diode, the second diode and the second switching element are connected to a bridge circuit, and one end of the reactor is connected to the junction of the first switching element and the first diode, and a remaining end of the reactor is connected to the junction of the second diode and the second switching element; and excitation mode in which the first switching element is turned on the second switching element is turned on, free-wheeling mode-1 in which the first switching element is turned off and the second switching element is turned on, the excitation mode, and free-wheeling mode-2 in which the first switching element is turned on and the second switching element is turned off are sequentially performed in a unit period T, and, when the control is terminated, demagnetization in which the first switching element and the second switching element are turned off is performed, hereby being able to reduce current ripple caused by the fast dynamic characteristics of the high-speed SRM and overcome overshoot current caused by the time delay of current sensing.
2. Description of the Related Art
In general, a switched reluctance motor is a kind of reluctance motor, and includes a multi-phase stator formed by winding an armature coil and configured to generate magnetic force, a rotor adapted to be rotated by magnetic force generated by a stator and magnetic force generated based on the relative positions of teeth, and a location detection unit configured to include a location sensing unit and a sensor plate and to sense the location of the rotor by detecting location detection pulses at predetermined angular resolution as the location of the rotor varies. In this switched reluctance motor, a plurality of teeth is symmetrically formed in the rotor, the armature coil is symmetrically wound around the multi-phase stator, and the location sensing unit senses the locations of the rotor and outputs location sensing pulses, thus sequentially driving the multi-phase armature coil in synchronization with the location sensing pulses.
In the switched reluctance motor, power applied to the armature coil wound around the multi-phase stator is selectively supplied and cut off using a switching element. A forward rotating torque corresponding to an input pulse signal may be generated in the rotor based on magnetic inhaling force by applying an input pulse signal to the control terminal of the switching element in synchronization with the location sensing pulse of the location sensing unit and, thereby, sequentially varying the excitation state between the rotor and the stator. When a specific excitation state is not varied, the rotor may be stopped in a specific location. A reverse rotating force may be generated by controlling the phase of an input pulse signal applied to the switching element on the basis of the maximum inductance shape. Accordingly, a variety of types of drive control can be achieved, so that the motor is being usefully used in various application fields.
In particular, a high-speed switched reluctance motor (SRM) has compact size and high system efficiency, so that it frequently applied to and used for high RPM systems, such as a blower, a compressor, and a pump. A common SRM control method is configured to control an SRM by controlling driving current in accordance with driving speed. In order to control driving current, Pulse Width Modulation (PWM) is used. As is well known, PWM is a method which is configured to perform modulation by changing the widths of pulses in response to the magnitude of modulation signals, and is widely used to control the current of an electric motor which is subjected to variable and constant speed driving.
In the meantime, the high-speed driving of an electric motor is subjected to a variety of types of limiting factors, such as iron loss, loss in winding, the length of the iron core of a stator, dynamics in a shaft, bearings, the VA (Voltage-Amps) rating of a controller. Accordingly, the design and control of the current of an SRM suitable for high-speed driving require many considerations. Furthermore, in a high-speed driving system, due to electrical frequency and core loss from a high frequency, the number of poles is a very important factor. In order to reduce an electrical frequency at the same speed, many high-speed drives adopt a two-pole system.
Meanwhile, the high-speed SRM is problematic in that the impedance of the motor is significantly smaller than that of a general motor so as to deal with fast electrical dynamic characteristics, with the result that current ripple increase in a high-speed operation condition compared to the conventional motor system. In order to reduce current ripple, high-frequency switching is appropriate. The switching frequency is normally limited by switching loss and the electrical characteristics of a power device.
Meanwhile, a high resolution encoder is not suitable for a high-speed motor due to its mechanical, electrical limitations. The sensing of the rotor location of a low resolution encoder is problematic in that it is not suitable to control phase current without incurring an overshoot phenomenon during the excitation interval of the switched reluctance motor due to the time delay of hardware and the feedback signal of phase current.
Accordingly, due to the fast rotation characteristics of the high-speed SRM, the delay time of current sensing may be a great problem and overshoot current resulting from the delay time is considered to be a serious problem.
A plurality of conventional technologies related to a method of controlling the current of an SRM has been proposed. An example of these technologies is Korean Patent No. 294,209. This conventional technology discloses an apparatus for controlling the current of a switched reluctance motor, the switched reluctance motor including an inverter unit for variably switching an excitation state with respect to each phase of a multi-phase stator, in which, when a current sensing unit is electrically connected to the inverter unit and senses current actually applied to an armature coil, a current control signal is generated to compensate for the current error between the current reference of a current reference unit and the actually applied current using a current control unit, and the current signal is received, subjected to pulse width modulation and then provided to the inverter unit, thereby being able to effectively improve the operating characteristics of the switched reluctance motor by minimizing current error.
This conventional technology is advantageous in that it enables optimal current control on the basis of the location of a rotor relative to that of a stator, so that the rotating torque of the motor can be maximized and noise generated by torque ripple during rotation can be reduced, thereby improving the overall efficiency of the switched reluctance motor. However, it is disadvantageous in that there is an actual limitation to its application to high-speed operation mode in light of the characteristics of the high-speed SRM, and, even if it is applied to high-speed operation mode, it is still difficult to reduce the current ripple of the high-speed SRM and overcome overshoot current using this control method.
Another conventional technology, that is, Korean Patent Application No. 228,695, discloses a method of controlling the driving time of a switched reluctance motor, which is configured to receive a location signal related to the sensed location of a rotor and to allow the motor to operate within delay time limits while maintaining a turn-on duration for a specific time on the basis of a Look-Up Table (LUT) in which preset speed-based turn-on delay time data is recorded for respective rotating speeds, thus being able to effectively reduce torque ripple by optimizing the turn-on and turn-off time of an armature coil.
However, this technology is still disadvantageous in that it is not suitable for high-speed operation mode, in that accuracy is low because a look-up table for delay time data, which can be inaccurately recorded due to various situations, is used, and in that it cannot reduce the current ripple of the high-speed SRM.